<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>z-index不灵</title>
    <style>
        div {
            height: 100px;
            width: 200px;
            color: #ffffff;
            background-color: red;
            position: absolute;
        }
    </style>
</head>
<body>
    <div>Red box</div>
    <select name="" id="">
        <option value="1">111</option>
        <option value="2">222</option>
        <option value="3">333</option>
    </select>
</body>
</html>